Multiple diffusion pump array for improved statistical pumping of vacuum work chamber



ZIQEIZI Aug. 26, 1969 RUSH 3,463,897

MULTIPLE DIFFUSION PUMP ARRAY FOR IMPRQVED STATISTICAL 1 PUMPING OF VACUUM WORK CHAMBER Filed April .17. 1967 2 Sheets-Sheet 1 FIG. 2

R OHM T. mw VR .5 MZ 0 H w A V. B

2 Sheets-Sheet 2 INVENTOR.

H. M. RUSH BY ATTORNEY H. M. RUSH Aug. 26, 1969 MULTIPLE DIFFUSION PUMP ARRAY FOR IMPROVED STATISTICAL PUMPING OF VACUUM WORK CHAMBER Filed April 17, 1967 FIG. 4

United States ABSTRACT on THE DISCLOSURE Increased efiiciency of diffusion pumping of a vacuum work chamber is achieved by reducing the distance be- 3,463,897 Patented Aug. 26, 1969 ice or to different work surface areas on the same large workpiece.

OBJECTS Among the several objects of the invention, principal objects reside in the provision of improvements in diffusion V pumping methods and arrangements which effect increases tween an available pump inlet and the gas molecules to be pumped thereby, and by placing 'aplurality of such pump inlets strategically about the chamber. Small pumps used for this purpose operate into a common manifold, and the vacuum work chamber and its pumps and manifold are thus operable together as a portable unit. An electron beam gun is positioned within the vacuum work chamber so as to perform a welding operation.

BACKGROUND This invention relates generally to methods of obtaining a high vacuum and more particularly to improvements in diffusion pumping methods and systems for achieving increased pumping efficiency in the development of vacuum environmental conditions suitable for the performance of such operations as brazing and electron beam welding within the vacuum chamber.

A diffusion pump removes gas from the vacuum chamber only because molecules of the gas happen to find their a function of flow probability P, that is, 'the statistical progression of molecules to the pump inlet. Values of P have been determined for variousconfigurations'such as pipes, elbows, baffles and valves, and must similarly be in'pumping efiiciency as well as increased utility'and portability of the affected vacuum chambers. Still other objects, features and advantages of the invention will become more fully apparent as the description proceeds, reference being had to the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 respectively are perspective and plan views in schematic form of vacuum chambers having multiple diffusion pump arrays constructed and arranged in accordance with the present invention; and

FIGS. 3 and 4, respectively, are plan and elevational views of an electron beam welder employing the multiple diffusion pump peripheral array of the present invention, certain parts being broken away for the sake of clarity.

THE SPECIFICATION Referring now to the drawings for a more complete understanding of the invention, and first more particularly to FIGS. 1 and 2 thereof, 10 designates a vacuum chamber having gas inlets 11 disposed about the periphery thereof. Small diameter diffusion pumps 12 connect respectively to the inlets 11. As best seen in FIG. 1, the pumps are disposed generally vertically and make connection with the chamber inlets through elbows 13.

The outlets of pumps 12 connects into a common manifold 14 which, in turn, connects as by the ducting 15 with a roughing or backing pump 16 which exhausts the pumped gas molecules to the atmosphere as indicated by the arrow 17 in FIG.

Manifold 14 is constructed in any suitable manner and is suitably mounted and supported on chamber 10. Outlet lines 18, FIG. 1, from pumps 12 are connected, as

determined, or estimated, in connection with the various system, present a number oflgas flow impeding surfaces which may reduce the flow probability factor to a low value, often less than 0.10;

In accordance with the present invention, multiple diffusion pump inlets are placed at strategic points around a chamber to be evacuated, and gas flow impeding surfaces disposed between the pump inlets and the gas molecules to be removed are reduced to a minimum, all to the end that the flow probability to each inlet P approaches unity. Small diameter diffusion pumps mounted on the vacuum chamber, as in a peripheral array for the purpose, are operated into a common manifold which, in turn, operates through flexible ducting into a backing or roughing pump. The chamber and small pumps and manifold mounted thereon together constitute a portable unit which may be moved, as required, as where the chamber may be open ended and releasably scalable to other equipment, or to the surface of a workpiece. Thus, for example, the portable chamber may be removable to render a vacuumbrazed panel assembly accessible for further processing, or the chamber may be moved in a succession of electron beam welding operations from workpiece to workpiece,

' indicated at 19, to the manifold 14.

Pumps 12 may be placed at any position around chamber 10. Selected locations are usually made in areas where there would be a clear line of sight from the pumps across the chamber. In small chambers, it may become necessary to specially arrange or cut away parts of internal struc tures in order to provide the desired clear line of sight. In the copending application of M. E. Gerard, Ser. No. 605,852, for 'Apply Type E.B. Welding Apparatus, for example, the supporting frame members for the gun traverse mechanism are cut away to provide for effective operation of the diffusion pumps whose inlets lie adjacent of these frame members.

Referring now to FIGS. 3 and 4, the electron beam welder shown therein comprises a vacuum chamber 20 which is substantially of cylindrical configuration. An electron beam gun 21 mounted within chamber 20 has an outlet orifice tube 23 through which the electron beam 24 extends to perform a work function on the surface of a schematically disclosed workpiece designated W. Workpiece W is disposed within a work chamber 25 which forms a releasably sealed engagement with the high vacuum chamber 20 at their common junction 26 which is releasably sealed as by the O-ring 27.

Chamber 20 has a viewing port 28 which provides a view of the welding operation as presented on the angularly mounted mirror 29.

In this instance, the diffusion pump inlets 11 are disposed near the top of the chamber 20 and substantially equidistant therearound to thus provide, at least in part, a line of sight diametrically across the top of the electron gun 21. Onlythe electrical supply connections 30 project H upwardly from the gun to impede thefiowalong' this line of sight.

Typically in accordance with the prior art arrangements, a single diffusion pump operable into'a roughing pump would be employed with the Welder of FIGS. 3 and 4. Assuming the prior art use of a 6 inch diffusion pump for the purpose, three 2 inch pumps would be the equivalent of the 6 inch-pump. The efficiency of the three pump array, however, is increased by 3 to 4 'times over that aiforded by the six inch pump. The measure of the improvement in the use of the multiple small difiusion pumps over the-use of a single large pump is by the greater efliciency which can be achieved..-

The rated speed of a typical 6-inch difiusion pump .is 1500 liters per second. Due, however, to the low value of the fiow probability factor incidental to the single pump From the foregoing, it should now be apparent that diffusion pump methods and arrangements have been distherein.

use, the efiiciency of the 6-inch pump would only be of the order of 10% to thus provide a net speed of only placed, the flow probability need be calculated at 75% conductance for a valve only, or for a valve and elbow 13, L

at approximatley 54% conductance. Thus, for three 2- inch pumps, the net pumping speed becomes:

3 x285 X 54% =46l.7 liters/sec.

as compared with 150 liters/sec. for the 6-inch pump.

. A multiple diffusion pump array as hereindisclosed is used to advantage in a double vacuum brazing system wherein the vacuum environment required for brazing is developed by the diffusion pump array mounted on a retort housing the panel assembly to be brazed, the retort, its pump array, and manifolding being disposed in a secondary vacuum chamber and connected to externally and remotely situated roughing and backing pumps.

In summary, the improved diffusion pumping method of the present invention comprises the steps of strategically placing a plurality of gas inlets on a chamber in which the gas flow is molecular-at random so that the distance between each inlet and the gas molecules to be pumped therefrom is minimized and the statistical progression of the gas molecules to each inlet provides a high flow probability value which approaches unity, diffusion pumping the gas molecules which respectively find their way to these inlets, and directing the pumped gas flow from these inlets into a common outlet. The system for practicing this method comprises a portable unit including the chamber, a multiple array of small diameter diifusion pumps mounted thereon respectively at the gas inlets, and a common manifold extended peripherally of the chamber and connected to the pumps, and a remotely stationed roughing and backing pump having a flexible connection with the manifold.

Having thus described the invention, what is claimed as new and useful and desired to be secured by Letters Patentis: v? I 1. A portable diifusion pumping system for a vacuum work chamber comprising, inlcombination, a high vacuum chamber .within which the gas flow is molecular-at random under conditions in which a work function is to beperformed upon a workpiece by means disposed therewith within the chamber, means-for heating a workpiece mounted within the vacuum work chamber, said chamber having a peripherally extending wall, a plurality of elbowshaped gas inlets mounted on said wall and strategically spaced thereabout in relation to the disposition of said workpiece and its associated work function performing means whereby the distance between each of said inlets and the gas molecules to be pumped thereby is minimized and the statistical progression of gas molecules to each inlet has a high probability value approaching unity, a plurality of small diameter diffusion pumps depended respectively from said plurality of elbow inlets and having outletsfor pumping away the gas molecule that find their way to the elbow inlets, a manifold mounted on said chamber wall and extended peripherally thereof, and a plurality of gas flow conduits respectively interconnecting said diffusion pump outlets with said manifold whereby the chamber together with its pumps and manifold are movable as a portable unit.

2. A portable system as in claim 1 and further comprising a roughing and backing pump and a [flexible elongated gas flow connection connecting said roughing and backing pump with said manifold thereby to render said chamber and its pumps and manifold movable as a portable unit with respect to said roughing and backing pump.

References Cited UNITED STATES PATENTS 8/1963 Smith 23044 10/1964 Orr 230101 

